1. Field of the Invention
The present invention pertains to an improved nozzle for use in wear-resistant rotary atomizers, to atomizer wheels containing said improved nozzles and to a method for employing said nozzles.
2. Discussion of the Prior Art
Wear-resistant rotary atomizers are described in Niro patents U.S. Pat. No. 3,454,226, U.S. Pat. No. 4,121,770 and U.S. Pat. No. 4,684,065. Those patents describe atomizer wheels for atomizing slurries of a highly abrasive material, comprising a wheel hub and a mainly cylindrical external wall defining an annular chamber of a substantially bowl-like cross-sectional shape coaxially surrounding said hub, a number of substantially horizontal and radial ejection apertures distributed over the circumference of said external wall. During operation the supplied slurry is ejected outwards through said ejection apertures in atomized form into a surrounding drying chamber in which the fine particles formed by the atomization are dried so that their content of solids drops down to the bottom of the drying chamber as a fine powder.
U.S. Pat. No. 3,454,226 describes nozzles of a wear-resistant sintered material arranged in each of said apertures fitting loosely with respect to said external wall, the nozzles projecting into said annular chamber. The use of wear-resistant sintered material for the nozzles was stated to be necessary because of the very hard wear which takes place on account of the very high velocities of discharge from the atomizer wheel caused by the centrifugal force when atomizing suspensions which contain solid particles of a hard material.
In U.S. Pat. No. 4,684,065 the nozzle of U.S. Pat. No. 3,454,226 is replaced by a nozzle assembly which comprises a lining of wear-resistant sintered material arranged in the apertures by means of replaceable steel bushings fitting loosely with respect to said external wall. The nozzle assembly is held in place by flexible sealing rings which also prevent liquid penetrating into the space between the aperture wall and the nozzle assembly. A flat recess is formed in the internal side of the bushing facing the lining arranged therein. Said flat recess extends in the axial direction of the bushing on either side. The claimed advantage of this design is that upon inevitable flexing of the atomizer wall under rotation, the bushing can deform without fracturing the brittle ceramic liner arranged therein.
The nozzles described in the above-mentioned patents and the commercially available nozzles have flow channels of essentially cylindrical shape, although in FIG. 4 of U.S. Pat. No. 3,454,226 also a nozzle having a square cross-section is depicted. The specific designs described in the prior art provide atomizer wheels and nozzles which are highly wear-resistant and have a long lifetime. The present invention provides atomizer wheels and nozzles having the same wear resistance as the ones described in the Niro patents or even better, but at the same time, the nozzles have been improved to provide microspherical particles with a more narrow particle size distribution.
In one embodiment, the present invention is a nozzle for a rotary atomizer comprising a flow channel in the form of a vertical slot.
In a second embodiment, the present invention is an atomizer wheel for atomizing slurries of a highly abrasive material, comprising a wheel hub and a mainly cylindrical external wall defining an annular chamber of a substantially bowl-like cross-sectional shape coaxially surrounding the hub. A number of substantially horizontal and radial ejection apertures are distributed over the circumference of the external wall. A nozzle comprised of a wear-resistant sintered material is arranged in each of said apertures and fitts loosely with respect to the external wall. The nozzle projects into the annular chamber and has a flow channel in the form of a vertical slot.
In a third embodiment, the present invention is a method of obtaining solid particles of relatively small particle size distribution. A slurry of solid material is atomized by ejecting the slurry through at least one ejection nozzle into a drying chamber in which particles of the solid material are formed by the atomization are dried and collected. The ejection nozzle comprises a flow channel in the form of a vertical slot.
Other embodiments of the invention lie in details concerning nozzle constriction, particularly with regard to the handling of abrasive solid material, and details concerning the method of obtaining the solid particles.